The present invention relates to methods and systems for administering a pharmaceutical gas to a patient and, more particularly, to methods and systems for introducing carbon monoxide CO or nitric oxide NO to a patient in a predetermined quantity.
The normal or conventional way of giving a pharmaceutical drug to a patient is to prescribe the dose based on the quantity of drug (usually in weight) per unit weight of the patient (e.g. mg/Kg) with the dose being specified to be delivered over a period of time or being repeated at specified intervals of time. This allows the user to control the quantity of drug and ensures the quantity of drug being delivered is in proportion to the patient's size. This is to reduce the patient to patient variability in response to the drug due to the size of the patient i.e. a 7 Kg baby will not get the same quantity of drug as a 80 Kg adult.
In recent times there have been a number of gases which have been shown to have pharmaceutical action in humans and animals. Examples include Nitric Oxide (NO) Zapol et al U.S. Pat. No. 5,485,827 and more recently Carbon Monoxide (CO) Otterbein et al (U.S. Published Patent Application No. 2003/0219496). In the Otterbein patent application, CO is described as having a pharmacological action in a number of medical conditions including ileus and vascular disease.
In these cases, the carbon monoxide gas needs to be delivered to the patients alveoli where it can move across the alveolar membrane and into the blood stream where its action can take effect. The current dosing used in these cases is for the patient to breath at a specified concentration of CO in ppm for a specified period of time. Accurate dosing of CO for these treatments is important as CO reacts with the hemoglobin in the blood to form carboxyhemoglobin which means the hemoglobin is no longer able to carry oxygen to the tissues of the body. If too much CO is given, the patient may exhibit the toxic effects of CO for which it is usually known.
There is a tight window for CO delivery between the therapeutic level and the level that causes carboxyhemoglobin above safe levels. Up until now CO has been delivered as a constant concentration in the gas breathed by the patient/animal for a specified period of time. For example in reference 3 of the Otterbein publication, (Example 2 pg 13) the therapeutic dose delivered to mice for the treatment of ileus was 250 ppm of CO for 1 hour.
However, this method of dosing CO can be associated with large variability in the actual dose being delivered to the animal/humans alveoli. This variability is because the quantity of CO being delivered to the animal/patient is dependent on a number of variables including, but not limited to, the patients tidal volume, respiratory rate, diffusion rate across the alveolar and ventilation/perfusion (V/Q) matching.
The amount of CO delivered into a patient's alveoli can be determined by the ideal gas law as shown in the following equation:N=PV/(RuT)  (1)
Where: N is the number of moles of the gas (mole) P is the absolute pressure of the gas (joule/m3) V is the volume of the particular gas (m3), Ru is the universal gas constant, 8.315 joule/mole-K and T is the absolute temperature (K).
If we assume atmospheric pressure (101,315 joule/m3) and 20° C. (293 K) as the temperature and we express the volume in mL (10−6 m3), then equation (1) reduces to:N=4.16×10−5V (moles)  (2)
Equation (2) can be used to calculate the number of moles of gas delivered to a patient's alveolar volume over a period of time when given a specified concentration by using the following equation:NCO=RR·t·CCO·10−6·4.16×10−5Va  (3)Where; CCO is the concentration of CO (ppm), Va is the alveolar volume (mL), RR is the respiratory rate (BPM) and t is the time in minutes.
For example, if the CO dose for ileus in humans was 250 ppm of CO for one hour (60 minutes), the alveolar volume is 300 mL and the patients respiratory rate is 12 breaths per minute (bpm) then the amount of CO gas in moles delivered to the patients alveoli over that period would be:NCO=12·60·250·10−6·4.16×10−5·300=2.25×10−3 moles
This can be converted into the mass of drug delivered (MCO) using the gram molecular weight of CO which is 28 as shown in the following equation:MCO=NCO·28=63×10−3 g=63 mg  (4)
However, although this works for a given set of assumptions, a spontaneous patient's respiratory rate can vary widely from perhaps 8 to 20 breaths per minute depending on circumstances and the patient's alveolar volume per breath can also vary significantly from say 200 to 400 mL depending on the metabolic need. These variables can have a dramatic effect on the amount of gaseous drug being delivered to the patient over the same period of time. For instance, if the patients respiratory rate was 8 bpm and the alveolar volume was 200 mL, the CO dose delivered to the patients alveoli would have been 27.8 (mg). Likewise if the patients respiratory rate was 20 bpm and the alveolar volume was 400 mL, then the dose delivered to the patients alveoli would have been 139.2 (mg) thus representing a five-fold difference in the amount of drug being delivered.
This means, in the example of CO, the quantity of gaseous drug a patient gets as measured in grams could vary substantially depending on the patient's ventilation pattern. For a dose based on constant concentration and time, the effect of these variables could mean that an individual patient could get significantly higher or lower doses of CO in grams and this could result in either high unsafe levels of carboxyhemoglobin or doses too low to be effective. Although not all the gaseous drug delivered to the alveoli will be taken up by the body's bloodstream (due to variables such as cardiac output and the diffusion coefficient of the gas) controlling the amount delivered to the alveoli takes away a major source of variability.
In addition, there is a need to administer NO to a patient in a predetermined quantity as described in “Cell-free hemoglobin limits nitric oxide bioavailability in sickle-cell disease”, Nature Medicine, Volume 8, Number 12, December 2002, pages 1383 et seq. This paper describes the use of inhaled NO to react with cell free hemoglobin to form plasma methemoglobin and so reduce the ability of the cell free hemoglobin in the plasma to consume endogenously produced NO (FIG. 5, page 1386). The quantity of NO delivered to the patient blood needs to be equivalent to the amount of cell free hemoglobin that is in the patients plasma. The amount of NO delivered to a sample of sickle cell patients was 80 ppm of NO for 1.5 hours. However, there was variability in the amount of methemoglobin produced in individual patients as shown by the error bars on FIG. 4b. So, in a similar way to the CO example, a known quantity of NO needs to be delivered to a patient to provide the desired therapeutic effect and again it is important to remove any variability of delivery because of differences in the individual patient's respiratory pattern.
Accordingly, it would be advantageous to have a system and method of introducing pharmaceutical gases (such as carbon monoxide and nitric oxide) that allows for the precise control of a known quantity of the pharmaceutical gas to be delivered to the patients alveoli and which is not subject to change based on the patients respiratory patterns.